1. Field of the Invention
The invention relates to a refractive projection objective for microlithography, which consists, in the direction of light propagation, of a first convexity, a waist, and a second convexity. Such refractive projection objectives are also termed “single-waist” systems
2. Description of the Related Art
Such single-waist systems are known, for example, from U.S. Pat. No. 60/160799, EP 1 061 396 A2, and from EP 1 139 138 A1 or WO 01/23933-WO 01/23935. It is already known from these documents that the first or the first two object-side lenses have negative refractive power. Furthermore it is known from these documents that the imaging quality can be increased by the use of aspherics. Since the resolution attainable with a projection objective increases in proportion to the image-side numerical aperture of the projection objective, and furthermore in proportion to the reciprocal of the exposure wavelength, the endeavor is predominantly to provide projection objectives with the greatest possible numerical aperture in order to increase the resolution.
U.S. Pat. No. 6,560,031 shows another refractive projection objective having one waist and a first, object-side lens having negative refractive power, wherein at least one lens which is arranged between the object plane and the waist comprises an aspherical surface.
International patent application PCT/E 02/04846 filed on May 3, 2002 by the applicant and claiming priority of DE 102 10 899.4 filed on Mar. 8, 2002 shows other single-waist systems allowing very high image-side numerical apertures NA≧0.85, where values of NA≧1 can be obtained e.g. if used in conjunction with an immersion liquid.
US patent application US 2004/0120051 A1 based on international patent application PCT/EP03/01651 filed on Feb. 9, 2003 and claiming priority of provisional application 60/360,845 filed on Mar. 1, 2002 shows another refractive projection objective having one defined waist and negative lenses on the object-side entry.
International patent application PCT/EP03/01954 filed on Feb. 26, 2003 and claiming priority of DE 102 10 899.4 filed on Mar. 8, 2002 and corresponding to U.S. patent application Ser. No. 10/379,809 filed on Mar. 6, 2003 shows refractive projection objectives for immersion lithography designed as single-waist systems.
US patent application US 2004/0004757 A1 filed on May 5, 2003 and claiming priority of PCT/EP02/04846 filed on May 3, 2002 and DE 102 24 361 filed on May 24, 2002 discloses very-high aperture projection objectives designed as single-waist systems having a belly (or convexity) near the object, a belly near the image and a waist defined therebetween, where a negative group which has an effective curvature with a concave side directed towards the image is arranged in a region of divergent radiation between the waist and the system diaphragm arranged in the belly near the image. The disclosure of this document is incorporated herein by reference.
It is furthermore necessary, with the requirements set on a projection objective in microlithography, to use selected materials of high quality. Fluoride materials are in particular only available to a limited extent at present in the required quality. For example, at an exposure wavelength of 193 nm, a few lenses of calcium fluoride are used for compensation of chromatic aberration in projection objectives designed for this wavelength. Furthermore, calcium fluoride lenses, which are not so sensitive as regards compaction, are preferably used close in front of the wafer.